fix: Ollama message sanitization and streaming think slugs

- Fix `invalid message content type: <nil>` error from Ollama
- Create TatlockOllamaProvider that sanitizes messages (null → "")
- Update all agents to use sanitized provider
- Fix repeating think messages by adding ReasoningSummaryDone signal

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/responses/streaming.py

@@ -211,8 +211,10 @@ class StreamingCoordinator:
                 )
 
                 # Stream think slugs that were collected during delegation
+                # Each think message is complete, so we signal done after each
                 for think_msg in orchestration_results.get("think_messages", []):
                     yield ReasoningSummaryDelta(delta=think_msg)
+                    yield ReasoningSummaryDone()
                     await asyncio.sleep(0.05)
 
             else: